When drilling, completing, or otherwise operating on a well, it is often necessary to determine if the material in the annular space between the formation and casing, and/or the annular space between multiple casing strings is filled with cement, fluids, mud and/or solid materials. For example, when cementing to provide zonal isolation, cement is usually placed in the annular space (by any one of various methods), displacing the material in the annular space with cement that will solidify with time. The original fluids in the annular space can include water, drilling mud, hydrocarbons, formation fluids, formation solids, and any type of combination of above. It is standard practice to log the well before completion to determine the efficiency with which these fluids are displaced and the consequent effectiveness of the zonal isolation. Several logging tools have been developed in the past to help determine the material behind pipe including sonic, ultrasonic and temperature tools, and they are often run together to evaluate the cementing job.
As another example, operators often wish to determine if the pipe or casing is attached to formation after the economic life of the well is finished. It may be possible to cut the casing above the attached point or points and remove the casing from the well. The same logging tools are run to determine the portions of casing that are free. From the data recorded from these cement evaluation tools both the original cement sheath, and the remaining cement sheath can be determined by scientific principles that are well known and available.
One of the common methods to determine the presence of cement sheath is the use of ultrasonic tools that provide a measurement of the acoustic impedance of the material, which is defined as Z=ρV, where V is the speed of sound in the material and ρ is the density of the material. However, there are regions where the acoustic impedances of light (foam) cement overlap with the acoustic impedance values of water and light drilling mud (see FIG. 1). Additional analysis (based on the measurement variance) has been used in past to separate out foamed cement from liquids when the impedance values are the same.
Technology has evolved, and there now exist heavy drilling muds having acoustic impedance values that overlap with solid (non-foamed) cement (see FIG. 1). Techniques currently available cannot reliably distinguish between overlapping impedance values to determine cement bonding efficiency. Thus, there exists a need for a method and system to determine the quality of the bond between the casing and the surrounding formation.